Published by Australian Academy of Science
Activity 1 | Toxic algal blooms – a sign of rivers under stress

How to test the influence of various fertilisers on algal growth

Fertilisers used on farms and suburban parks and lawns contain phosphorus and nitrogen which wash into our waterways. Compare how different fertilisers affect the growth of algae.

Materials (for the class)

pond water (source of algae)
distilled water
large glass jars
measuring cylinder
fertilisers (choose some that are high in nitrogen and others that are high in phosphorus)
tall, thin glass container with a flat bottom
newspaper

Procedure

1. Set up a number of large glass jars equal to the number of fertilisers you wish to test, plus one jar which will act as the control.

2. To each jar add 1 litre of distilled water and 100 millimetres of pond water.

3. Do not add fertiliser to the control jar. To each of the sample jars add the concentration of fertiliser recommended on the respective packets.

4. Let all the jars stand in a well-lit position for about 4 weeks.

5. Compare the amount of algal growth by comparing the turbidity (cloudiness) of the water. This can be done by placing a jar on some newsprint. Repeat with the other samples, including the control, and compare the difficulty in reading the newsprint through the top of the jar.

6. Do any algae stick to the sides of the jar? How would you include them in your measurement of algal growth?